The present invention relates to a pressure gauge and more particularly to a gauge for measuring the difference in pressure between two pressure signals and an associated pressure relieving feature.
Differential pressure can be determined in many ways. One of the simplest methods of determining differential pressure is to measure or observe the motion of a close fitting piston balanced by a spring in a cylinder. A higher pressure is applied to the end of the cylinder not in communication with the spring and a lower pressure is applied to the end of the cylinder in communication with the spring. Accordingly, the higher pressure will cause the piston to move. The spring will be caused to compress until the force required to compress the spring added to the force caused by the lower pressure equals the force caused by the higher pressure. The distance that the piston travels can be measured and when carefully calibrated in terms of pressure differential will provide a reading of the difference between the high and low pressures.
Differential pressure gauges of the spring-balanced piston type are subject to erratic operation due to the disposition and collection of contamination in the clearance space between the piston and the associated cylinder. Typically, the clearance of the piston and the cylinder is very small (approximately 0.0003 inches) so that there is very little fluid flow possible. No seal is used because it would create friction thereby decreasing the accuracy of the gauge.
A test for possible contamination in a gauge that has a cylinder made of glass is commonly performed. The test is typically achieved by closing the low pressure connection and then venting the cavity beneath the piston so that the high pressure on top of the piston will drive the piston over its full length of travel. The operator observes the piston motion to detect erratic movement which would be indicative of binding caused by contaminating deposits. To provide for this test, it has been customary to use a 3-way selector valve located at the port where the low pressure connection is made.
Another issue which must be considered in the operation of equipment that is subject to pressure is to provide for protection against excessive pressure by utilizing a pressure relief valve. A common application for a pressure relief valve is on a filter vessel in a pipeline system. If the pipeline valves are closed in a way to isolate the filter vessel, solar heating causes an increase in temperature of the fluid in the vessel resulting from thermal expansion of the fluid which is far greater than the expansion of the materials used to make the vessel. The pressure relief valve releases enough fluid to prevent a pressure that is greater than the design pressure of the vessel.
If a differential pressure gauge is installed on the vessel, it is protected from excessive pressure by the pressure relief valve on the vessel. However, if isolation valves are closed which are installed in the tubing connecting the differential pressure gauge to the vessel, the excessive pressure in the gauge cannot be relieved. The gauge will break if the pressure exceeds the design pressure parameters of the gauge. To avoid this problem, a separate pressure relief valve can be installed on the gauge at considerable cost.
It is an object of the invention to produce a gauge for measuring a pressure differential between two pressure signals including an automatically operative pressure relief valve.
Another object of the invention is to produce a gauge for measuring a pressure differential between two pressure signals including a manually operable spring return push button to provide a venting function useful for inspecting the operation of the pressure operated piston of the gauge.
The above, as well as other objects of the invention, may be readily achieved by a differential pressure gauge having an automatically operative pressure relief valve and a manually operable three-way valve to provide a venting function useful for inspecting the operation of the pressure operated piston of the pressure gauge.